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High-energy particles reveal volcanic interiors

Flowing magma could one day be traced beneath the surface of active volcanoes using instruments that detect the high-energy space particles that stream through the rock.

Scientists have shown that muons – high-energy particles generated when cosmic rays interact with the Earth’s atmosphere – can be used to probe the inner structure of volcanoes.

They appear to be able to trace geological activity as it occurs and might one day provide early warnings of a volcanic eruption. Existing methods for probing volcanoes – such as echo-sounding – would be unreliable by comparison.

Muons created in the atmosphere speed off in all directions and pass through the Earth at a rate of 10,000 per square metre every minute. They pass harmlessly through ordinary matter but are deflected slightly depending on the density of the material traversed.

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Active duty

Nagamine Kanetada and colleagues at the KEK Muon Science Laboratory in Japan, are conducting experiments using the particles to probe two active Japanese volcanoes – Mount Asama, on Honshu Island, and Mount Kurokura, south-west of Tokyo.

Detectors placed on the side of the volcanoes record muons as they pass through. A computer model then calculates how many particles should be detected if different types of rock are below the surface, making it possible work out what geological features lie inside the mountains.

At the moment, the sensors can only be used to peer up to two metres below the surface. But better detectors, particularly ones that record the direction of travel of the muon, should increase this sensitivity dramatically, the team says.

Magma and water

During the Mount Asama study, which began in January 2002, the researchers discovered a cavity inside the volcano, estimated to contain about 30% magma. They will soon return to the same spot to see if this has changed since an eruption occurred in the summer of 2004. At Mount Kurokura, the detector spotted changes that could correspond to water flow below the surface.

Kanetada says the experiments so far are a proof of principle and adds that increased sensitivity would yield even more valuable geological information. “If we could get twenty times as much data we could see magma flow over a few days,” he told the annual American Association for the Advancement of Science meeting in Washington, DC.

It is an inherently difficult trick to pull off, however, as muon sensors are sensitive to background cosmic ray electrons and gamma rays. To solve the problem, Kanetada developed a sensor with two components – one facing the mountain and another facing the sky, making it possible to identify and eliminate unwanted background noise.

Nuclear materials

Muons promise to become a valuable remote-sensing tool in several other areas as well. US researchers at Los Alamos National Laboratory plan to use muon detectors to scan shipping containers as they enter the US. They could signal the presence of high-density nuclear materials which could be used to build crude nuclear weapons.

“The scattering of muons is very sensitive to the density and atomic number of materials,” explains Christopher Morris at LANL.

And Arturo Menchaca-Rocha and colleagues at the National Autonomous University of Mexico in Mexico City hope to detect the presence of a royal burial chamber or sacrificial sarcophagus hidden in the Sun Pyramid in Teotihuacan.